Novel imido polymers

ABSTRACT

Imido prepolymers and thermally stable imido polymers, well adapted for the production of a wide variety of useful molded or laminated shaped articles, or cellular materials, are prepared by reacting, at elevated temperatures, (a) at least one N,N&#39;-bismaleimide, with (b) at least one aromatic di(meth)acrylate, in the presence of a catalytically effective amount of (c) an imidazole.

This application is a continuation of application Ser. No. 387,772,filed Aug. 1, 1989, now abandoned, which is a continuation ofapplication Ser. No. 112,460 filed Oct. 26, 1987 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel thermally stable polymersproduced from bisimides.

2. Description of the Prior Art

Polymers produced by reacting an N,N'-bisimide of an unsaturateddicarboxylic acid such as, for example, an N,N'-bismaleimide, with adiprimary diamine are described in French Patent No. 1,555,564. Thequantities of N,N'-bisimide and of diamine are selected such that theratio: ##EQU1## is at least equal to 1, and preferably is less than 50.Thermally stable resins are thus produced which withstand severe thermalstresses remarkably well.

The above-mentioned French patent also discloses that the resins may beprepared in bulk by heating intimate admixture of the reactants. Theseresins may also be prepared in an inert polar diluent such asdimethylformamide, N-methylpyrrolidone or dimethylacetamide. The latterprocess is the preferred, when the end application of the polymerrequires it to be in solution form.

For many uses, the noted '564 French patent indicates that a two-stepprocess is advantageous. In a first stage, a prepolymer is prepared byheating the intimate admixture of the two reactants to a temperature onthe order of 100° to 250° C. The prepolymer thus produced may beemployed in solution, suspension or powder form, or else may simply beshaped by casting while in hot state. In a second stage, curing of theprepolymer is effected by heating it to temperatures on the order of300° C., under pressure if desired.

These polymers may be converted into films or into polycellularmaterials. They are of very special interest for the preparation ofmolded shaped articles, in combination, if desired, with fibrous orpowdered fillers or laminates based on inorganic fibers (single fibers,fiber cloth or nonwovens) such as, for example, carbon, boron or glassfibers. However, the preparation and use of such polymers require strictprecautions to be observed for reasons of health and hygiene when thediprimary diamine is aromatic in nature, because of the toxicityassociated with certain of such aromatic diamines.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofnovel imido polymers, the production of which does not entail the use ofdiamines.

Briefly, the present invention features polymers containing imidegroups, prepared by reacting, at a temperature ranging from 50° C. to300° C.:

(a) at least one N,N'-bisimide having the formula: ##STR1## in which thesymbols Y, which are identical or different, each denote H, CH₃ or Cl,the symbol A denotes a divalent radical selected from amongcyclohexylene, phenylene, 4-methyl-1,3-phenylene,2-methyl-1,3-phenylene, 5-methyl-1,3-phenylene,2,5-diethyl-3-methyl-1,4-phenylene, and the radicals of the formula:##STR2## in which T denotes a single valence bond or a group: ##STR3##and the symbols X, which are identical or different, each denote ahydrogen atom or a methyl, ethyl or isopropyl radical;

with (b) at least one aromatic diacrylate having the formula: ##STR4##in which the symbol B denotes a divalent radical of the formula:##STR5## in which the symbol U denotes a single valence bond or a group:##STR6## and the symbols n, which are identical or different, eachdenote a number equal to zero, 1, 2, 3, 4 or 5, in the presence of:

(c) an imidazole compound.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

More particularly according to the present invention, exemplary of thebisimides having the formula (I), representative are:

N,N'-Meta-phenylenebismaleimide,

N,N'-Para-phenylenebismaleimide,

N,N'-4,4'-Diphenylmethanebismaleimide,

N,N'-4,4'-Diphenyl ether bismaleimide,

N,N'-4,4'-Diphenyl sulfone bismaleimide,

N,N'-1,4-Cyclohexylenebismaleimide,

N,N'-4,4'-Diphenyl-1,1-cyclohexanebismaleimide,

N,N'-4,4'-Diphenyl-2,2-propanebismaleimide,

N,N'-4,4'-Triphenylmethanebismaleimide,

N,N'-2-Methyl-1,3-phenylenebismaleimide,

N,N'-4-Methyl-1,3-phenylenebismaleimide,

N,N'-5-Methyl-1,3-phenylenebismaleimide.

These bismaleimides may be prepared according to the process describedin U.S. Pat. No. 3,018,290 and British Patent No. 1,137,290.N,N'-4,4'-Diphenylmethanebismaleimide, either alone or admixed withN,N'-2-methyl-1,3-phenylenebismaleimide,N,N'-4-methyl-1,3-phenylenebismaleimide and/orN,N'-5-methyl-1,3-phenylenebismaleimide are preferably used to carry outthe present invention.

As specific examples of diacrylates of the formula (II), particularlyrepresentative are the diacrylates and the dimethacrylates of thefollowing diphenols, whether or not di-(mono- or polyoxyethylated):

4,4'-Dihydroxydiphenylmethane,

Bisphenol A,

4,4'-Dihydroxydiphenyl ether.

Diacrylates and dimethacrylates of di(mono- or polyoxyethylated)bisphenol A are preferably selected. The compounds (b) which are moreparticularly preferred are the diacrylate (or the dimethacrylate) ofdi(monooxyethylated) bisphenol A and the diacrylate (or thedimethacrylate) of di(dioxyethylated) bisphenol A [cf. formula (II) inwhich the symbol B denotes the radical: ##STR7## and n is equal to 1 and2].

The quantities of N,N'-bisimide(s) (a) and of diacrylate(s) (b) areselected such that the ratio r: ##EQU2## ranges from 1.7/1 to 20/1, andpreferably from 2/1 to 8/1.

The imidazole compound (c) corresponds to the general formula: ##STR8##in which R₁, R₂, R₃ and R₄, which are identical or different, eachdenote a hydrogen atom, an alkyl or alkoxy radical containing from 1 to20 carbon atoms, or a vinyl, phenyl or nitro radical, with the provisothat R₃ may form, together with R₄ and the carbon atoms from which theydepend, a single ring member such as, for example, a benzene ring.

Exemplary of such imidazole compounds, representative are, inparticular: imidazole or glyoxaline, 1-methylimidazole,2-methylimidazole, 1,2-dimethylimidazole, 1-vinylimidazole,1-vinyl-2-methylimidazole and benzimidazole.

The imidazole compound is employed in catalytically effective amounts.Depending on the nature of the imidazole compound and depending on thepolymerization rate desired during the application stage, the imidazolecompound is employed in a proportion, expressed as the number of molesof compound (c) per 100 g of the mixture bisimide(s) (a)+diacrylate(s)(b), situated in the range of from 0.15×10⁻³ to 6×10⁻³, and preferablyfrom 0.4×10⁻³ to 4×10⁻³.

The polymers according to the invention may be prepared in bulk, bydirectly heating the bisimide(s) (a), the acrylate reactant (b) and theimidazole compound (c), at least until a homogenous liquid mixture isobtained. The temperature may vary depending on the physical state ofthe compounds present, but it generally ranges from 50° C. to 300° C. Itis advantageous to have and to maintain the starting material compoundsin an intimately mixed state, both before and during the heating, forexample, by means of effective stirring. The imidazole compound (c) ispreferably added at the beginning of the operation to a well-stirredmixture of the reactants (a) and (b) such as to permit it to be rapidlydispersed. When this compound is particularly active, it is desirable toadd it in a solvent or diluent which is compatible with the reactionmixture, to avoid its encapsulation in the polymer network which isformed. It has been found that it may be advantageous to employ one ofthe polar organic liquids referred to below as a solvent or diluent.

The preparation of the polymers according to the invention may also becarried out by heating the mixture of the reactants in an organicdiluent which is liquid over at least a part of the range 50° C.-300° C.Exemplary of such diluents, particularly representative are the aromatichydrocarbons such as xylenes and toluene, halogenated hydrocarbons suchas chlorobenzenes, polar solvents such as dioxane, tetrahydrofuran anddibutyl ether, dimethylformamide, dimethyl sulfoxide,N-methylpyrrolidone, dimethylacetamide, methylglycol and methyl ethylketone. The polymer solutions or suspensions may be used as such fornumerous applications. The polymers may also be isolated, for example byfiltration, if desired after precipitation by means of an organicdiluent which is miscible with the solvent employed. In this connection,a hydrocarbon whose boiling point does not markedly exceed 120° C. maybe employed to advantage.

It should be appreciated that the properties of the polymers accordingto the invention may vary widely, depending especially on the precisenature of the reactants employed, on the proportions of reactants whichare selected and on the precise temperatures within the abovementionedrange. Insofar as the resultant polymers are concerned, these may becured polymers which are insoluble in the usual solvents such as, forexample, those liquids mentioned in the preceding paragraph, and whichdo not exhibit any appreciable softening below the temperature at whichthey begin to decompose.

However, these polymers may also be in the form of prepolymers (P) whichare soluble in polar organic solvents and have a softening point at atemperature below 200° C. (this softening point typically ranges from50° to 150° C.). These prepolymers may be produced in bulk by heatingthe mixture of reactants until a homogeneous or pasty material isobtained, at a temperature generally ranging from 50° to 180° C. for aperiod of time which may range from a few minutes to several hours (thistime period is proportionately shorter, the higher the temperature whichis selected). Before the mixture of reactants is heated, it isadvantageous, in this case as well, to convert it into an intimatemixture by stirring beforehand. This also constitutes a preferredembodiment for addition of the imidazole compound (c), namely, thatindicated above in connection with the direct preparation of curedpolymers. The preparation of the prepolymers may also be carried out insuspension or in solution in a diluent which is liquid at least in apart of the range 50°-180° C.

The prepolymers (P) may be employed in the form of a liquid mass, simplecasting while hot being sufficient for shaping and producing moldedarticles. It is also possible, after they have been cooled and ground,to use them in the form of powders which are remarkably well suitablefor compression molding applications, if desired in the presence offillers (powders, spheres, granules, fibers or flakes, etc.). In theform of suspensions or solutions, the prepolymers (P) may be used forthe production of preimpregnated intermediate articles, thereinforcement of which comprising fibrous materials (in the form ofsingle fibers, or woven or nonwoven sheets) based on aluminum orzirconium silicate or oxide, carbon, graphite, boron, asbestos or glass.These prepolymers (P) may also be employed for the production ofcellular materials after a blowing agent such as, for example,azodicarbonamide, has been added.

In a second stage, the prepolymers (P) may be cured by heating up totemperatures on the order of 300° C., generally from 150° to 300° C. Anadditional shaping operation may be carried out during the cure, undervacuum or at a superatmospheric pressure if desired, it also beingpossible for such operations to be carried out consecutively.

In a preferred embodiment of the present invention, the operation iscarried out in two stages, the first stage including heating the mixtureof the reactants from 50° C. to 180° C. to form a prepolymer (P), andthe second stage including curing the prepolymer (P), after the shapingthereof, by heating same up to temperatures on the order of 300° C.

However, in a more preferred embodiment of the present invention, theoperation is carried out in two stages, but by using, in the firststage, a continuous process for preparing the prepolymer (P), comprisingseparately introducing the bisimide(s) (a) in a divided solid state, thediacrylate(s) (b) in a liquid or molten state, and the imidazolecompound (c) in a solid state or in solution, into a mixer equipped withan extruder screw.

By the expression "mixer equipped with an extruder screw" is intendedapparatus which does not form any dead zones as the material advances.Machines of this type, which may include one or more screws, aredescribed in E. G. Fisher, Extrusion of Plastics (IntersciencePublishers, 1964), pages 104 to 108. These mixers may contain twoendless screws intermeshing intimately and rotating in the samedirection. Apparatus of this type, adapted more particularly for thepreparation of alkali metal terephthalates, is described in FrenchPatent No. 1,462,935. Another variety of mixers which can be employedincludes machines containing an endless screw with interrupted flightsproducing a rotary motion and an oscillating motion simultaneously inthe axial direction, and housed in a barrel comprising teeth whichinteract with the interrupted fins of the screw. Machines of this typeare described in French Patent Nos. 1,184,392, 1,184,393, 1,307,106 and1,369,283.

For reasons of convenience in use, it is preferable to employ thebisimide(s) (a) in the form of particles, the size of which ranges from0.1 to 5 mm. Introduction of such particulates into the mixer may beregulated by known means, such as metering screws or balances.

The diacrylate(s) (b) are introduced into the mixing zone in liquidstate. Their introduction may be carried out by means of a meteringpump. Introduction of the reactant (b) may be conducted at one or morelocations situated preferably downstream of the bisimide(s) (a) inletzone.

The imidazole compound (c) may be incorporated in the solid state withthe acrylate reactant (b). However, it is preferable to incorporate theimidazole compound (c) in the form of a solution in a polar solvent,such as those discussed above. In this preferred embodiment, theimidazole compound (c) may very well be incorporated with the acrylatereactant (b), but its introduction into the mixer may also beaccomplished at any other location in the mixing zone, preferablydownstream of the acrylate reactant (b) inlet zone.

The maintaining of the mixing zone at the selected temperature of from50° C. to 180° C., and preferably from 130° C. to 160° C., is typicallyaccomplished by controlled heating of the housing of the mixer employed.In addition, it is also possible to carry out a controlled heating ofthe endless screw or screws in the machine. Insofar as the housing isconcerned, the heating may be applied uniformly throughout its length,but it is also possible to arrange a number of adjoining heating zonesproviding the mixing zone with a temperature which may, for example,increase in the direction of travel of the material. It is preferablefor the temperature to be in the range 20°-130° C. upstream of the pointof initial introduction of the reactant (b).

The residence time of the materials in the mixing zone may vary to acertain degree depending on the bisimide(s) (a) employed, on thetemperature selected and on the weight ratio of the reactants. As ageneral rule, it is on the order of 1 to 30 minutes. At the outlet ofthe mixer, the softening point of the prepolymer (P) can be regulated byheating the latter in an oven under determined temperature and durationconditions. For example, when the reactant (a) isN,N'-4,4'-diphenylmethanebismaleimide and the ratio r is close to 5/1,prepolymers which have softening points on the order of 50° C. to 60° C.are obtained using a prepolymerization temperature of 155° C. and aresidence time within the range of 5-10 minutes.

The polymers according to the invention are of interest for industrialapplications which require materials having good mechanical andelectrical properties, together with a high chemical inertness attemperatures of from 200° to 300° C. By way of examples, they aresuitable for the manufacture of plate or tubular insulators forelectrical transformers, printed circuit boards, pinions, rings, and thelike. The preimpregnated articles can be used for producing componentsof various shapes and for various purposes in many industries such as,for example, in aeronautics. These components, known as laminates, whichmay be solids of revolution, are produced by arraying several layers ofprepregs onto a form or a support. The prepregs may also be used asreinforcements or as means for repairing worn components. It will berecalled that to produce molded articles, for example, it is possible tobegin either with the mixture of the reactants (a)+(b)+(c), or with aprepolymer (P). When directly beginning from the mixture of thereactants, this mixture is given the shape of the required article andthe curing is then effected by heating. When beginning with theprepolymer (P), this may be molded by simple casting while in hot state,or by injection molding and its cure is then effected by heating.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

This example describes production of a polymer according to theinvention by a noncontinuous process.

Into a reactor equipped with a stirrer, and placed in an oil baththermostated at 160° C., were introduced:

(i) 300 parts by weight of N,N'-diphenylmethanebismaleimide, having asoftening point of 155° C.; and

(ii) 100 parts by weight of di(dioxyethylated) bisphenol A diacrylate;this compound is available commercially from UCB under the registeredtrademark Ebecryl 150.

The mixture was stirred for 30 minutes until a homogeneous mass wasobtained. 0.28 parts by weight of imidazole were then added in the formof a 17% by weight solution in N-methylpyrrolidone.

Stirring of the reaction mixture was continued for 45 minutes at 160°C., and the mixture was then cast on a tray. After cooling, theresulting prepolymer was ground and a yellow powder which was soluble insolvents such as, for example, N-methylpyrrolidone anddimethylformamide, was obtained. The prepolymer had a softening point of80° C.; its viscosity, measured as a 50% by weight solution inN-methylpyrrolidone, was 2.5 poises.

A solution of prepolymer, 50% by weight in N-methylpyrrolidone, wasemployed to coat a glass cloth (manufactured by Porcher under thereference 7628) which had a weight per unit area of 200 g/m² and whichhad been subjected to a treatment with gamma-aminopropyltriethoxysilane(Union Carbide silane A 1100). The impregnated cloth contained 35 g ofprepolymer per 65 g of cloth. It was dried in a ventilated atmospherefor 5 minutes at 130° C. 6 squares (15×15 cm) were then cut therefromand were stacked, with a 35 μm thick copper sheet placed on one of theouter face surfaces of the stack, and the assembly was placed betweenthe platens of a press under the following conditions:

Pressure: 40×10⁵ Pa,

Heating of the press platens: 15 minutes at 150° C., then 45 minutes at200° C.

During the lamination operation, a degree of polymer creep of about 20%by weight was observed. After a thermal finishing treatment for 6 hoursat 235° C., the adhesion of the copper to the 6-layer laminate wasexamined. This adhesion, measured with a tensometer by pulling thecopper at an angle of 90° (according to the standard MIL P 55 617 B witha pull speed of 55 mm/min) was on the order of 15 N/cm. This value wasmaintained after an aging period of 1,000 hours at 200° C.

As a comparative test, the above Example 1 was reproduced, but in theabsence of imidazole. When heated to 160° C., the reaction mixture setsolid after 1 hour 30 minutes of mixing. After cooling and grinding, thepowder obtained had a softening point of 70° C. Its solubility in theabovementioned solvents was very low. Analysis showed that thebismaleimide had reacted with the diacrylate only to a very slightextent or not at all. The prepolymer obtained in this manner wasunsuitable for the applications described, in particular for thepreparation of laminates.

EXAMPLE 2

This example describes production of a polymer in accordance with theinvention by a continuous process.

The apparatus employed was a Buss laboratory mixer designated as"Ko-Kneader" model PR 46. This mixer comprised an endless screwconsisting of a shaft fitted with interrupted helical flights, with theinterruptions defining separate fins. It was driven by an appropriatemechanism. The screw was housed in a body comprising three adjoiningjacketed coaxial cylindrical barrels; the inner wall of the mixer bodywas provided with tooth-shaped projections. The screw was driven with arotary motion and simultaneously with an oscillating motion in its axialdirection, and this created an exchange of material in two directions.

Water, at 20° C., was circulated in the first jacket, and a fluid heatedto 155° C. in the other two. The speed of rotation of the screw was 60revolutions/minute.

N,N'-4,4'-diphenylmethanebismaleimide was introduced into the first partof the mixer (corresponding to the first barrel) by means of a meteringbalance at a rate of 1,172 g/hr. The bismaleimide was introduced in theform of particles, the average size of which was on the order of 0.25mm.

The mixture of di(dioxyethylated) bisphenol A diacrylate and imidazolesolution (in accordance with Example 1) was introduced into the secondpart of the mixer (corresponding to the second barrel), at a rate of 340g/hr.

The reactants were metered such that the bismaleimide represented 77.5%of the weight of the bismaleimide/diacrylate mixture and the imidazole0.06% of the weight of the same mixture.

The average residence time of the material in the mixer was on the orderof 6 minutes, 30 seconds. A prepolymer whose softening point was on theorder of 55° C. was collected at the outlet of the machine. When heatedfor 17 minutes to 170° C., this prepolymer had a softening point of 80°C. It was soluble in solvents such as, for example, N-methylpyrrolidoneand dimethylformamide, and it had a viscosity of 2.5 poises (measured asindicated in Example 1).

The prepolymer obtained in this manner was used to prepare prepregs andlaminates comprising 18 covers (18 layers of prepregs) under theconditions described above in Example 1 (it should be noted that nocopper sheet was employed in this case). After another cure for 6 hoursat 235° C., the laminates had the following mechanical characteristics:

Flexural strength measured at 200° C. (according to French standard NF T51 001): 340 MPa,

Flexural modulus: 14,000 MPa.

Furthermore, the prepolymer obtained was subjected to TMA measurements(thermal mechanical analysis; ASTM standard E 831-81 with a rate oftemperature increase of 5° C./min). The product cured again at 235° C.and was observed to have a mean expansion coefficient of 50 μm/m/°C.between 40° C. and 300° C.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. An imido polymer essentially consisting of thepolymerizate of:(a) at least one N,N'-bisimide having the formula:##STR9## in which the symbols Y, which are identical or different, areeach H, CH₃ or Cl; the symbol A is cyclohexylene, phenylene,4-methyl-1,3-phenylene, 2-methyl-1,3-phenylene, 5-methyl-1,3-phenylene,2,5-diethyl-3-methyl-1,4-phenylene, or a radical of the formula:##STR10## in which T is a single valence bond or a group: ##STR11## andthe symbols X, which are identical or different, are each a hydrogenatom or a methyl, ethyl or isopropyl radical, with (b) at least onearomatic diacrylate of the formula: ##STR12## in which the symbol B is adivalent radical of the formula: ##STR13## wherein the symbol U is asingle valence bond or a group: ##STR14## and the symbols n, which areidentical or different, are each a number equal to zero, 1, 2, 3, 4 or 5wherein the ratio between the number of moles of the at least oneN,N'-bisimide (a) and the number of moles of the at least one aromaticdiacrylate (b) ranges from 1.7/1 to 20/1; said polymerizate being formedin the presence of: (c) a catalytically effective amount of an imidazolecompound.
 2. The imido polymer as defined by claim 1, said at least oneN,N'-bisimide (a) comprising N,N'-4,4'-diphenylmethanebismaleimide oradmixture thereof with N,N'-2-methyl-1,3-phenylenebismaleimide,N,N'-4-methyl-1,3-phenylenebismaleimide and/orN,N'-5-methyl-1,3-phenylenebismaleimide.
 3. The imido polymer as definedby claim 1, said at least one aromatic diacrylate (b) comprising adiacrylate or dimethacrylate of a di(polyoxyethylated) bisphenol A. 4.The imido polymer as defined by claim 1, said ratio ranging from 2/1 to8/1.
 5. The imido polymer as defined by claim 1, in thermally stablethermoset state, solvent insoluble, and exhibiting no appreciablesoftening below its degradation temperature.
 6. The imido polymer asdefined by claim 1, in thermosetting prepolymer state, soluble in polarorganic solvents, and having a softening point at a temperature of lessthan 200° C.
 7. A shaped article comprising the imido polymer as definedby claim
 5. 8. A shaped article comprising the imido polymer as definedby claim
 6. 9. The shaped article as defined by claim 8, comprising aprepreg.
 10. The imido polymer as defined by claim 1, said imidazolecompound having the formula: ##STR15## in which R₁, R₂, R₃ and R₄, whichare identical or different, are each a hydrogen atom, an alkyl or alkoxyradical containing from 1 to 20 carbon atoms, or a vinyl, phenyl ornitro radical, with the proviso that R₃ may form, together with R₄ andthe carbon atoms from which they depend, a single hydrocarbon ringmember.
 11. The imido polymer as defined by claim 10, said imidazolecompound comprising imidazole or glyoxaline, 1-methylimidazole,2-methylimidazole, 1,2-dimethylimidazole, 1-vinylimidazole,1-vinyl-2-methylimidazole or benzimidazole.